Detection of loss of neutral

ABSTRACT

A method for detecting a break in connection of a neutral of a three-phase electricity network, implemented in a processing unit of an item of electrical equipment connected to the electricity network includes acquiring, at a time T, a first phase voltage (V 1 ), a second phase voltage (V 2 ) and a third phase voltage (V 3 ) measured by voltage sensors of the item of electrical equipment; evaluating a first quantity representative of a ratio between a maximum voltage and a minimum voltage from the first, second and third phase voltages; if the first quantity is greater than a predetermined threshold: evaluating, based on the first, second and third phase voltages, a second quantity; detecting a break in the neutral at the time T when the second quantity satisfies a predetermined reference criterion.

The invention relates to the field of electrical power distributionnetworks and equipment connected to said networks.

BACKGROUND OF THE INVENTION

An electricity distribution network is used to transport electricalpower from a power generation unit to one or more connected electricalinstallations. Electrical power is generally transported in three-phaseform, the electricity network then being made up of three phaseconductors and one neutral conductor. An electricity network is alsotypically equipped with a plurality of electricity meters used tomeasure the electrical power consumed by the connected electricalinstallations.

In a three-phase electricity network, a break in the neutral connectionmay occur upstream of one or more meters. Such a break is theresponsibility of the energy supplier in charge of the network and cancause major problems at the connected electrical installations. Indeed,depending on the load impedances of the connected electricalinstallations (downstream of the break point of the neutral), asignificant imbalance of the phase voltages carried by the three phaseconductors may occur. It is therefore possible for high voltages to bepresent at connected electrical installations, which may possiblydestroy said installations.

It is therefore important to be able to reliably, simply and quicklydetect a sudden break in the neutral connection in a three-phaseelectricity distribution network. This allows preventive and/orprotective measures to be taken quickly for connected electricalinstallations.

Conventionally, the detection of a break in the connection of a neutralis based on the detection of an abnormal imbalance between the phasevoltages of the distribution network and/or an absence of currentflowing in the neutral conductor. However, this method is generallyunreliable as it does not correctly identify whether the observedimbalance is related to a break in the neutral or to a break in one ormore phases. Furthermore, detecting the absence of current through theneutral conductor requires the use of current sensors positioned on saidneutral conductor, which increases costs and gives rise to physical andelectrical implementation problems.

Another known method for detecting a break in the connection of aneutral is to measure the downstream load impedances (of the connectedelectrical installations) in order to determine the expected imbalanceof the phase voltages in the event of a break in the neutral connection(and therefore to detect it when said imbalance actually occurs).However, the effectiveness and reliability of this method are directlyrelated to the reliability of the downstream load impedancemeasurements. This is a limiting factor, because determining theexpected imbalance between the phase voltages typically uses atheoretical analysis for which the downstream load impedances areassumed to be linear and constant over time (which is not necessarilythe case in reality). Moreover, in the event that the break in neutralaffects several electricity meters, each having distinct (uncorrelated)and unbalanced downstream load impedances, the method described abovecan no longer detect the break in the neutral connection. Indeed, for agiven meter, the measured downstream load impedance only considers theelectrical installations downstream of this meter. However, theimbalance between the phase voltages is linked to the equivalentdownstream load impedance resulting from the combination of all thedownstream load impedances of all the meters affected by the break inthe neutral connection. Since the equivalent load impedance is notmeasurable, the method described above is no longer applicable in thiscase.

OBJECT OF THE INVENTION

The object of the invention is a method for detecting, in an electricitydistribution network, a break in the connection of a neutral upstream ofone or more electricity meters in a quick, simple and reliable manner.

SUMMARY OF THE INVENTION

In order to achieve this object, a method for detecting a break inconnection of a neutral of a three-phase electricity network isproposed, the detection method being implemented at least partially in aprocessing unit of an item of electrical equipment connected to theelectricity network, and comprising the steps, repeated regularly, of:

-   -   acquiring, at a time T, a first phase voltage measured between a        first phase of the three-phase electricity network and the        neutral, a second phase voltage measured between a second phase        and the neutral, and a third phase voltage measured between a        third phase and the neutral, the first, second and third phase        voltages being measured by voltage sensors of the item of        electrical equipment;    -   evaluating a first quantity representative of a ratio between a        maximum phase voltage and a minimum phase voltage from the        first, second and third phase voltages;    -   if the first quantity is greater than a predetermined threshold:        -   evaluating, based on the first, second and third phase            voltages, at least a second quantity representative of a            current balance between said first, second and third phase            voltages;        -   detecting a break in connection of the neutral at the time T            when the at least one second quantity satisfies a            predetermined reference criterion.

The detection method according to the invention is thereforeparticularly advantageous because it makes it possible to detect a breakin the connection of a neutral in a distribution network (upstream of anelectricity meter) from the simple measurement of the phase voltagescarried by the phase conductors of said network. The detection method istherefore simple to implement (because it only requires voltage sensors)and inexpensive.

Furthermore, the detection method according to the invention is alsohighly reliable because, when there is a suspected break in the neutral(i.e., when the first quantity is greater than the predeterminedthreshold), the second quantity is evaluated and makes it possible toconfirm without any possible doubt that a break in the neutralconnection has indeed occurred.

Since the second quantity is evaluated directly on the basis of thephase voltages (the second quantity is evaluated only on the basis ofthe first, second and third phase voltages and therefore does notrequire any additional measurements), the detection method according tothe invention can quickly detect a break in the neutral connection.

Moreover, a detection method as previously described is proposed, inwhich the at least one second quantity comprises a second quantity thatis a function of a sum of pairwise products of root mean square valuesof the first, second and third phase voltages.

${{G2_{G2}} = {\sqrt{\frac{1}{3}\left( {{V_{1{eff}}V_{2{eff}}} + {V_{2{eff}}V_{3{eff}}} + {V_{3{eff}}V_{1{eff}}}} \right)}{Moreover}}},$

a detection method as previously described is proposed, in which saidsecond quantity is equal to:

${{G2_{G2}} = \sqrt{\frac{1}{3}\left( {{V_{1{eff}}V_{2{eff}}} + {V_{2{eff}}V_{3{eff}}} + {V_{3{eff}}V_{1{eff}}}} \right)}},$

-   -   where V_(1eff), V_(2eff) and V_(3eff) are respectively a root        mean square value of the first phase voltage, a root mean square        value of the second phase voltage and a root mean square value        of the third phase voltage,    -   BorneInf≤G2≤BorneSup the predetermined reference criterion being        that:    -   BorneInf≤G2≤BorneSup.

Moreover, a detection method as previously described is proposed,further comprising the steps of:

-   -   detecting whether:

V _(1eff) =V _(nom) and V _(2eff) =V _(nom) and V _(3eff) =V _(nom),

-   -   where V_(nom) is a nominal root mean square value of the phase        voltage of the electricity network;

${BorneInfBorneSupBorneInf} = {{V_{nom}{BorneSup}} = {\sqrt{\frac{{2\sqrt{3}} + 1}{4}}V_{nom^{-}}}}$

if this condition is met, defining and as follows:

${BorneInfBorneSupBorneInf} = {{V_{nom}{BorneSup}} = {\sqrt{\frac{{2\sqrt{3}} + 1}{4}}V_{nom}{and}}}$${BorneInfBorneSupBorneInf} = {{V_{nom}{BorneSup}} = {\sqrt{\frac{{2\sqrt{3}} + 1}{4}}{V_{nom}.}}}$

-   -   V_(1eff)=α₁·V_(nom) V_(2eff)=α₂·V_(nom) V_(3eff)=α₃·V_(nom)        BorneInf=min(B1, B2, B3, B4, B5, B6)BornSup=max (B1, B2, B3, B4,        B5, B6)Moreover, a detection method as previously described is        proposed, in which, if said condition is not met, and if:    -   V_(1eff)=α₁·V_(nom) V_(2eff)=α₂·V_(nom) V_(3eff)=α₃·V_(nom)        BorneInf=min(B1, B2, B3, B4, B5, B6)BornSup=max(B1, B2, B3, B4,        B5, B6) and and, where a₁, a₂ and a₃ are real coefficients such        that:    -   V_(1eff)=α₁·V_(nom) V_(2eff)=α₂·V_(nom) V_(3eff)=α₃·V_(nom)        BorneInf=min(B1, B2, B3, B4, B5, B6)BornSup=max(B1, B2, B3, B4,        B5, B6) and    -   V_(1eff)=α₁·V_(nom) V_(2eff)=α₂·V_(nom) V_(3eff)=α₃·V_(nom)        BorneInf=min(B1, B2, B3, B4, B5, B6)BornSup=max(B1, B2, B3, B4,        B5, B6),    -   where

${{B1} = {\frac{\sqrt{\left( {a_{1}^{2} + a_{2}^{2} + {a_{1}a_{2}}} \right)\left( {a_{1}^{2} + a_{3}^{2} + {a_{1}a_{3}}} \right)}}{3}V_{nom}}},$${{B2} = {\frac{\sqrt{\left( {a_{2}^{2} + a_{3}^{2} + {a_{2}a_{3}}} \right)\left( {a_{2}^{2} + a_{1}^{2} + {a_{2}a_{1}}} \right)}}{3}V_{nom}}},$${{B3} = {\frac{\sqrt{\left( {a_{3}^{2} + a_{1}^{2} + {a_{3}a_{1}}} \right)\left( {a_{3}^{2} + a_{2}^{2} + {a_{3}a_{2}}} \right)}}{3}V_{nom}}},$${{B4} = {\sqrt{\frac{\begin{matrix}{{2\sqrt{\left( {{4a_{1}^{2}} + a_{2}^{2} + a_{3}^{2} + {2a_{1}a_{2}} + {2a_{1}a_{3}} - {a_{2}a_{3}}} \right)\left( {a_{2}^{2} + a_{3}^{2} + {a_{2}a_{3}}} \right)}} +} \\\left( {a_{2}^{2} + a_{3}^{2} + {a_{2}a_{3}}} \right)\end{matrix}}{12}}V_{nom}}},$ ${{B5} = {\sqrt{\frac{\begin{matrix}{{2\sqrt{\left( {{4a_{2}^{2}} + a_{3}^{2} + a_{1}^{2} + {2a_{2}a_{3}} + {2a_{2}a_{1}} - {a_{3}a_{1}}} \right)\left( {a_{3}^{2} + a_{1}^{2} + {a_{3}a_{1}}} \right)}} +} \\\left( {a_{3}^{2} + a_{1}^{2} + {a_{3}a_{1}}} \right)\end{matrix}}{12}}V_{nom}}},$ ${B6} = {\sqrt{\frac{\begin{matrix}{{2\sqrt{\left( {{4a_{3}^{2}} + a_{1}^{2} + a_{2}^{2} + {2a_{3}a_{1}} + {2a_{3}a_{2}} - {a_{1}a_{2}}} \right)\left( {a_{1}^{2} + a_{2}^{2} + {a_{1}a_{2}}} \right)}} +} \\\left( {a_{1}^{2} + a_{2}^{2} + {a_{1}a_{2}}} \right)\end{matrix}}{12}}{V_{nom}.}}$

Moreover, a detection method as previously described is proposed, inwhich the at least one second quantity comprises a second quantity thatis a function of an area of an actual triangle formed by the first,second and third phase voltages in the Fresnel diagram.

$A = {{{\frac{1}{2}\left( {{V_{1{eff}}V_{2{eff}}\sin\varphi_{12}} + {V_{2{eff}}V_{3{eff}}\sin\varphi_{23}} + {V_{3{eff}}V_{1{eff}}\sin\varphi_{31}}} \right)A_{ref}} - \varepsilon_{1}} \leq A \leq {A_{ref} + \varepsilon_{1}}}$

Moreover, a detection method as previously described is proposed, inwhich the area of the actual triangle is determined by using theformula: where A is the second quantity, V_(1eff), V_(2eff) and V_(3eff)are respectively the root mean square values of the first, second andthird phase voltages, and φ₁₂ is a first phase shift between the firstphase voltage and the second phase voltage, φ₂₃ is a second phase shiftbetween the second phase voltage and the third phase voltage and φ₃₁ isa third phase shift between the third phase voltage and the first phasevoltage, the reference criterion then being that the second quantity issuch that:

$A = {\frac{1}{2}\left( {{V_{1{eff}}V_{2{eff}}\sin\varphi_{12}} + {V_{2{eff}}V_{3{eff}}\sin\varphi_{23}} + {V_{3{eff}}V_{1{eff}}\sin\varphi_{31}}} \right)}$

-   -   where A_(ref) is an area of a reference triangle and ε₁ is a        first predetermined measurement uncertainty.

Moreover, a detection method as previously described is proposed, inwhich, if the first, second and third phase voltages are perfectlybalanced, the area of the predetermined reference triangle is evaluatedby using the formula:

${A_{ref} = {\frac{3\sqrt{3}}{4}V_{nom}}},$

where A_(ref) is the area of the reference triangle and V_(nom) is thenominal root mean square value of the phase voltage of the electricitynetwork.

U₁₂U₂₃ U₃₁Φ₁−ε₂≤U₁₂≤Φ₁+ε₂−ε₂≤U₂₃≤Φ₂+ε₂Φ₃−ε₂≤U₃₁≤Φ₃+ε₂ Moreover, adetection method as previously described is proposed, in which the atleast one second quantity comprises second quantities which comprise afirst line-to-line voltage representative of a difference between thefirst phase voltage and the second phase voltage, a second line-to-linevoltage representative of a difference between the second phase voltageand the third phase voltage and a third line-to-line voltagerepresentative of a difference between the third phase voltage and thefirst phase voltage, the reference criterion then being that:

U ₁₂ U ₂₃ U ₃₁Φ₁−ε₂ ≤U ₁₂≤Φ₁+ε₂Φ₂−ε₂ ≤U ₂₃≤Φ₂+ε₂Φ₃−ε₂ ≤U ₃₁≤Φ₃+ε₂ and

U ₁₂ U ₂₃ U ₃₁Φ₁−ε₂ ≤U ₁₂≤Φ₁+ε₂Φ₂−ε₂ ≤U ₂₃≤Φ₂+ε₂Φ₃−ε₂ ≤U ₃₁≤Φ₃+ε₂ and

U ₁₂ U ₂₃ U ₃₁Φ₁−ε₂ ≤U ₁₂≤Φ₁+ε₂Φ₂−ε₂ ≤U ₂₃≤Φ₂+ε₂Φ₃−ε₂ ≤U ₃₁≤Φ₃+ε₂ and

U₁₂U₂₃ U₃₁Φ₁−ε₂≤U₁₂≤Φ₁+ε₂Φ₂−ε₂≤U₂₃≤Φ₂+ε₂Φ₃−ε₂≤U₃₁≤Φ₃+ε₂ where Φ₁, Φ₂ andΦ₃ are reference values of the first, second and third line-to-linevoltages measured during operation at a reference time T₀ preceding thetime T and ε₂ is a second predetermined measurement uncertainty.

Moreover, a detection method as previously described is proposed, inwhich the at least one second quantity comprises second quantities thatcomprise a first phase shift between the first phase voltage and thesecond phase voltage, a second phase shift between the second phasevoltage and the third phase voltage and a third phase shift between thethird phase voltage and the first phase voltage, the predeterminedreference criterion then being that the first, second and third phaseshifts are each non-zero and different to 120 degrees.

Moreover, a detection method as previously described is proposed,further comprising the step of detecting a break in the neutralconnection when it has been detected that the second quantity satisfiesthe predetermined reference criterion a predetermined number of times,corresponding to consecutive instances it is satisfied, spaced apart twoby two in time by a predetermined duration.

Moreover, a detection method as previously described is proposed, inwhich, when a break in the neutral connection has been detected, themethod further comprises the step of generating an alarm signal that canbe timestamped in a memory of the item of electrical equipment and/orthat can be transmitted to an item of equipment external to said item ofelectrical equipment.

Also proposed is an item of electrical equipment comprising voltagesensors and a processing unit arranged to implement the detection methodas previously described.

Also proposed is an item of electrical equipment as previouslydescribed, the item of electrical equipment being an electricity meter.

Also proposed is a computer program comprising instructions that causethe item of electrical equipment as previously described to perform thesteps of the detection method as previously described.

Also proposed is a computer-readable storage medium on which thepreviously described computer program is stored.

The invention shall be better understood in the light of the followingdescription of specific and non-limiting embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Reference is made to the appended drawings in which:

FIG. 1 shows a three-phase electricity distribution network;

FIG. 2 shows a Fresnel diagram of the phase voltages of the distributionnetwork shown in FIG. 1 in nominal conditions;

FIG. 3 shows an electronic architecture of the voltage sensors of anelectricity meter of the distribution network shown in FIG. 1 ;

FIG. 4 shows the influence of a break in the connection of a neutral onthe phase voltages of the distribution network shown in FIG. 1 ;

FIG. 5 shows the influence of a break in the connection of a neutral onthe phase voltages of the distribution network shown in FIG. 1 in theFresnel diagram;

FIG. 6 shows the steps of the method for detecting a break in theconnection of a neutral according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

In reference to FIG. 1 , an electricity distribution network 1 is shown.The distribution network 1 allows electrical power to be transportedfrom a power generation unit 2 to one or more electrical installations,in this instance a first electrical installation 3 and a secondelectrical installation 4.

The distribution network 1 is a three-phase electricity network thatcomprises a first phase conductor 5A, a second phase conductor 5B, athird phase conductor 5C and a neutral conductor 6. For the sake ofsimplicity, the term “conductor” shall be omitted hereinafter in thedescription, with reference being made simply to a first phase 5A, asecond phase 5B, a third phase 5C and a neutral 6.

The distribution network 1 carries a first phase voltage V₁ between thefirst phase and the neutral 6, a second phase voltage V₂ between thesecond phase 5B and the neutral 6 and a third phase voltage V₃ betweenthe third phase 5C and the neutral 6. The first, second and third phasevoltages V₁, V₂, V₃ are alternating (sinusoidal) voltages with afrequency of 50 Hz.

FIG. 2 shows the first phase voltage V₁, the second phase voltage V₂ andthe third phase voltage V₃ in the Fresnel diagram when the distributionnetwork 1 is operating in nominal conditions.

The root mean square value of the first phase voltage V_(1eff), the rootmean square value of the second phase voltage V_(2eff) and the root meansquare value of the third phase voltage V_(3eff) are all equal to thenominal root mean square value of the phase voltage, identified asV_(nom), which is equal to 230V.

Moreover, the first, second and third phase voltages V₁, V₂, V₃ are eachout of phase with one another. Therefore, a first phase shift φ₁₂ ispresent between the vector representation of the first phase voltage V₁and that of the second phase voltage V₂, a second phase shift φ₂₃ ispresent between the vector representation of the second phase voltage V₂and that of the third phase voltage V₃ and a third phase shift φ₃₁ ispresent between the vector representation of the third phase voltage V₃and that of the first phase voltage V₁. The first phase shift φ₁₂, thesecond phase shift φ₂₃ and the third phase shift φ₃₁ are all equal to120°.

FIG. 3 also shows a first line-to-line voltage U₁₂ which corresponds toa difference between the first phase voltage V₁ and the second phasevoltage V₂, a second line-to-line voltage U₂₃ which corresponds to adifference between the second phase voltage V₂ and the third phasevoltage V₃ and a third line-to-line voltage U₃₁ which corresponds to adifference between the third phase voltage V₃ and the first phasevoltage V₁.

In reference once more to FIG. 1 , a first meter 7 is connected to thedistribution network 1 between the generation unit 2 and the firstinstallation 3. The first meter 7 is intended to measure the consumptionof electrical power supplied to the first installation 3 via thedistribution network 1. The first installation 3 is in this instancerepresented by a first electrical impedance Z₁ connected between thefirst phase 5A and the neutral 6, by a second electrical impedance Z₂connected between the second phase 5B and the neutral 6 and by a thirdelectrical impedance connected between the third phase 5C and theneutral 6. Therefore, during operation in nominal conditions, the firstimpedance Z₁ has the first phase voltage V₁ at its terminals, the secondimpedance Z₂ has the second phase voltage V₂ at its terminals and thethird impedance Z₃ has the third phase voltage V₃ at its terminals.

A second meter 8 is also connected to the distribution network betweenthe generation unit 2 and the second installation 4. The second meter 8is intended to measure the consumption of electrical power supplied tothe second installation 4 via the distribution network 1.

The first meter 7 and the second meter 8 are three-phase meters.

The architecture of the first meter 7 will now be described.

The first meter 7 comprises a processing unit 7A which comprises atleast one processing component which may be a DSP (Digital SignalProcessor), a processor, a microcontroller, an FPGA (Field-ProgrammableGate Array) or an ASIC (Application-Specific Integrated Circuit).

The first meter 7 further comprises a memory 7B connected to theprocessing unit 7A or integrated into the processing unit 7A. The memory7B forms a computer-readable storage medium, on which at least onecomputer program is stored comprising instructions for at leastpartially implementing the detection method described below.

The first meter 7 further comprises a communication module 7C connectedto the processing unit 7A and arranged to transmit data using PLC (PowerLine Communication) technology. It should be noted that othercommunication standards may be used, cellular radio technology, forexample.

The first meter 7 further comprises voltage sensors 7D connected to theprocessing unit 7A and arranged to measure the first phase voltage V₁,the second phase voltage V₂ and the third phase voltage V₃ of thedistribution network 1.

The voltage sensors 7D of the first meter 7 will now be described ingreater detail in reference to FIG. 3 .

The voltage sensors 7D comprise a first branch 10, a second branch 11and a third branch 12.

The voltage sensors further comprise a first analogue-to-digitalconverter 13, a second analogue-to-digital converter 14 and a thirdanalogue-to-digital converter 15. In order to simplify the description,they will be referred to hereinafter as the first ADC 13, the second ADC14 and the third ADC 15.

The first branch 10 comprises two first resistors R_(1A), R_(1B). Thefirst resistor R_(1A) comprises a first terminal 10A connected to thefirst phase 5A and a second terminal 10B connected to an input 13A ofthe first ADC 13. The first resistor R_(1B) comprises a first terminal10C connected to the input 13A of the first ADC 13 and a second terminal10D connected to the neutral 6. The two first resistors R_(1A), R_(1B)thus form a voltage divider bridge between the first phase 5A and theneutral 6. A first measurement voltage V_(1m) is therefore presentbetween the input 13A of the first ADC 13 and the neutral 6. The firstmeasurement voltage V_(1m) is representative of the first phase voltageV₁ and is consistent with the input voltage range of the first ADC 13.

The second branch 11 comprises two second resistors R_(2A), R_(2B). Thesecond resistor R_(2A) comprises a first terminal 11A connected to thesecond phase 5B and a second terminal 11B connected to an input 14A ofthe second ADC 14. The second resistor R 2B comprises a first terminal11C connected to the input 14A of the second ADC 14 and a secondterminal 11D connected to the neutral 6. The two second resistorsR_(2A), R_(2B) thus form a voltage divider bridge between the secondphase 5B and the neutral 6. A second measurement voltage V_(2m) istherefore present between the input 14A of the second ADC 14 and theneutral 6. The second measurement voltage V_(2m) is representative ofthe second phase voltage V₂ and is consistent with the input voltagerange of the second ADC 14.

The third branch 12 comprises two third resistors R_(3A), R_(3B). Thethird resistor R_(3A) comprises a first terminal 12A connected to thethird phase 5C and a second terminal 12B connected to an input 15A ofthe third ADC 15. The third resistor R_(3B) comprises a first terminal12C connected to the input 15A of the third ADC 15 and a second terminal12D connected to the neutral 6. The two third resistors R_(3A), R_(3B)thus form a voltage divider bridge between the third phase 5C and theneutral 6. A third measurement voltage Vim is therefore present betweenthe input 15A of the third ADC 15 and the neutral 6. The thirdmeasurement voltage V_(3m) is representative of the third phase voltageV₃ and is consistent with the input voltage range of the third ADC 15.

The voltage divider bridges of each of the first, second and thirdbranches 10, 11, 12 make it possible respectively to reduce theamplitudes of the first, second and third phase voltages V₁, V₂, V₃ inorder to make them compatible with the measurement range of the firstADC 13, the second ADC 14 and the third ADC 15.

The first ADC 13 comprises an output 13B connected to the processingunit 7A. The first ADC 13 therefore produces first measurement samplesrepresentative of the first phase voltage V₁ and provides them to theprocessing unit 7A.

The second ADC 14 comprises an output 14B connected to the processingunit 7A. The second ADC 14 will therefore produce second measurementsamples representative of the second phase voltage V₂ and provide themto the processing unit 7A.

The third ADC 15 comprises an output 15B connected to the processingunit 7A. The third ADC 15 will therefore produce third measurementsamples representative of the third phase voltage V₃ and provide them tothe processing unit 7A.

It is assumed that the first ADC 13, the second ADC 14 and the third ADC15 all have suitable characteristics (number of bits, samplingfrequency) to correctly convert the first measurement voltage V_(1m),the second measurement voltage V_(2m) and the third measurement voltageV_(1m), respectively. In this instance, the first ADC 13, the second ADC14 and the third ADC 15 each have a number of bits greater than or equalto 12 bits and a sampling frequency of at least 2 ksps (kilo samples persecond).

The influence of a break in the connection of the neutral 6 in thedistribution network 1 will now be described.

In reference to FIG. 4 , it is assumed that a break in the connection ofthe neutral 6 occurs in the distribution network 1 at a break point 16situated upstream of the first meter 7. The term “upstream” should beunderstood to mean on the generation unit 2 side, and the term“downstream” should be understood to mean on the first installation 3side.

When the break in the connection of the neutral 6 occurs, a node 17becomes a floating node.

The first phase voltage V₁ downstream of the break point 16 thenbalances spontaneously according to the first impedance Z₁ of the firstinstallation 3. The first impedance Z₁ therefore has the first phasevoltage V₁ at its terminals, which is the voltage carried by the firstphase 5A downstream of the break point 16. The first phase voltage V₁ isexpressed as function of a first nominal phase voltage V₁₀ (which is thevoltage carried by the first phase 5A upstream of the break point 16)and a difference in potential V_(ne) induced by the break in theconnection of the neutral 6:

V ₁ =V ₁₀ −V _(ne).

Similarly, the second phase voltage V₂ downstream of the break point 16balances spontaneously according to the second impedance Z 2 of thefirst installation 3. The second impedance Z 2 therefore has the secondphase voltage V₂ at its terminals, which is the voltage carried by thesecond phase 5B downstream of the break point 16. The first phasevoltage V₂ is expressed as function of a second nominal phase voltageV₂₀ (which is the voltage carried by the second phase 5B upstream of thebreak point 16) and the difference in potential V_(ne):

V₂=V₂₀— V_(ne).

Similarly, the third phase voltage V₃ downstream of the break point 16balances spontaneously according to the third impedance Z₃ of the firstinstallation 3. The third impedance Z₃ therefore has the third phasevoltage V₃ at its terminals, which is the voltage carried by the thirdphase 5C downstream of the break point 16. The third phase voltage V₃ isexpressed as function of a third nominal phase voltage V₃₀ (which is thevoltage carried by the third phase 5C upstream of the break point 16)and the difference in potential V_(ne):

V ₃ =V ₃₀ −V _(ne).

It should be noted that, when the neutral 6 is connected correctly,V₁=V₁₀, V₂=V₂₀ and V₃=V₃₀.

In reference to FIG. 5 , the Fresnel diagram shows the influence of thebreak in connection of the neutral 6.

It is interesting to note that the break in the connection of theneutral 6 causes the first phase voltage V₁, the second phase voltage V₂and the third phase voltage V₃ to rebalance with each other. Except inthe specific scenario in which the first, second and third electricalimpedances Z₁, Z₂, Z₃ are perfectly balanced in the first, second andthird phases 5A, 5B, 5C, the new balance will be different from theinitial balance (when the neutral 6 was connected correctly).

The new balance involves a variation in the root mean square value ofthe first phase voltage V_(1eff), the root mean square value of thesecond phase voltage V_(2eff) and the root mean square value of thethird phase voltage V_(3eff) which are then no longer necessarily equalto the nominal root mean square value of the phase voltage V_(nom).

The new balance also involves a variation in the first phase shift φ₁₂,the second phase shift φ₂₃ and the third phase shift φ₃₁ which are thenno longer all necessarily equal to 120°.

However, the new balance retains the first line-to-line voltage U₁₂, thesecond line-to-line voltage U₂₃ and the third line-to-line voltage U₃₁.

In reference to FIG. 6 , the steps of the method for detecting a breakin the connection of a neutral according to the invention is nowdescribed.

The detection method according to the invention is implemented in theprocessing unit 7A of the first meter 7. More particularly, thedetection method according to the invention is implemented continuouslyby the processing unit 7A of the first meter. The steps of saiddetection method are therefore repeated regularly over time.

The method starts at a time T by acquiring the first phase voltage V₁between the first phase 5A and the neutral 6, the second phase voltageV₂ between the second phase and the neutral 6 and the third phasevoltage V₃ between the third phase 5C and the neutral 6 (step E1).

More specifically, the processing unit 7A acquires the first measurementsamples representative of the first phase voltage V₁, the secondmeasurement samples representative of the second phase voltage V₂ andthe third measurement samples representative of the third phase voltageV₃. The first measurement samples, the second measurement samples andthe third measurement samples are transmitted to the processing unit 7Aby the voltage sensors 7D.

Using the first measurement samples, the second measurement samples andthe third measurement samples, respectively, the processing unit 7Adetermines (possibly by applying calibration parameters) a root meansquare value of the first phase voltage V_(1eff), a root mean squarevalue of the second phase voltage V_(2eff) and a root mean square valueof the third phase voltage V_(3eff). The root mean square valuesV_(1eff), V_(2eff) and V_(3eff) are in this instance determinedrespectively by the processing unit 7A each second based on the first,second and third measurement samples acquired the previous second. Theroot mean square values V_(1eff), V_(2eff) and V_(3eff) could also bedetermined over a sliding second or over any other appropriate timewindow.

The processing unit 7A next defines and/or updates a predeterminedreference criterion (step E2). The predetermined reference criterionwill be described below.

It should be noted that step E2 is optional. Indeed, the predeterminedreference criterion may be determined a single time during theinstallation of the first meter 7 in the distribution network 1 (thepredetermined reference criterion being stored in the memory 7B). Thepredetermined reference criterion could also be defined and/or updatedduring maintenance of the first meter 7.

The processing unit 7A then evaluates a first quantity representative ofa ratio between a maximum phase voltage and a minimum phase voltage fromthe first, second and third phase voltages V₁, V₂, V₃ (step E3). Morespecifically, the first quantity is in this instance a number M that isdetermined by using the following formula:

$M = {\frac{\max\left( {V_{1{eff}},V_{2{eff}},V_{3{eff}}} \right)}{\min\left( {V_{1{eff}},V_{2{eff}},V_{3{eff}}} \right)}.}$

Evaluating the first quantity M makes it possible to check whether ornot there is an imbalance between the first, second and third phasevoltages V₁, V₂, V₃.

To this end, the first quantity M is compared to a predeterminedthreshold (step E4). The predetermined threshold is in this instanceequal to 1.2 in order to take into account, in particular, normaldispersions of the first, second and third phase voltages V₁, V₂, V₃(intrinsic in the distribution network 1). If the first quantity M isgreater than the predetermined threshold, there is a significantimbalance between the first, second and third phase voltages V₁, V₂, V₃,and this constitutes a first indicator of a suspected break in theconnection of the neutral 6 in the distribution network 1 upstream ofthe first meter 7.

However, the simple fact that the first quantity M is greater than thepredetermined threshold is not sufficient to be certain that a break inconnection of the neutral 6 has actually occurred. In particular, asimple break in the connection of one phase (from the first, second andthird phase 5A, 5B, 5C) could also lead to the same observation (thefirst quantity M is greater than the predetermined threshold). It istherefore necessary to confirm the suspected break in the connection ofthe neutral 6 by determining an indicator that is characteristic of abreak in the connection of a neutral in a three-phase electricitynetwork.

It should be noted that, if the first quantity M is less than thepredetermined threshold, the detection method according to the inventiondetermines that there is no break in the connection of the neutral 6,and therefore returns to step E1.

If the first quantity is greater than the predetermined threshold, theprocessing unit 7A evaluates, based on the first, second and third phasevoltages V₁, V₂, V₃, at least one second quantity representative of acurrent balance between said first, second and third phase voltages(step E5). Indeed, as shown in FIG. 5 , the break in the connection of aneutral gives rise to a new balance between the first, second and thirdphase voltages V₁, V₂, V₃.

The processing unit 7A then detects whether the at least one secondquantity satisfies the predetermined reference criterion.

Several methods are now possible and, in particular, several differentsecond quantities as well as several predetermined reference criteriacan be defined in order to effectively confirm that a break in theconnection of the neutral 6 has occurred.

According to a first embodiment, the at least one second quantitycomprises a second quantity G2 that is a function of a sum of thepairwise products of the first, second and third phase voltages V₁, V₂,V₃. More specifically, the second quantity G2 is a function of a sum ofpairwise products of root mean square values of the first, second andthird phase voltages V_(1eff), V_(2eff), V_(3eff) The approach proposedhere is based on the fact that, when there is a suspected break in theneutral connection, the first, second and third phase voltages V₁, V₂,V₃ become balanced according to the new balance, which is not completelyrandom.

If one of the root mean square values of a phase voltage from the rootmean square values of the first, second and third phase voltagesV_(1eff), V_(2eff), V_(3eff) decreases sharply, while the other two rootmean square values remain constant, this does not indicate a break inthe connection of the neutral 6. For example, if the root mean squarevalue V_(1eff) decreases sharply while the root mean square valuesV_(2eff) and V_(3eff) remain constant, it is rather a question of avoltage dip in the first phase 5A or a break in the connection of saidfirst phase 5A. However, if one of the root mean square values of aphase voltage from the root mean square values of the first, second andthird phase voltages V_(1eff), V_(2eff), V_(3eff) decreases sharply and,at the same time, one of the two other root mean square values increases(or indeed the two root mean square values increase) it is highlyprobable that this is linked to a break in the connection of the neutral6. For example, if the root mean square value V_(1eff) decreases sharplyand the root mean square values V_(2eff) and V_(3eff) simultaneouslyincrease, it is highly likely that this is linked to a break in theconnection of the neutral 6.

Similarly, if one of the root mean square values of a phase voltage fromthe root mean square values of the first, second and third phasevoltages V_(1eff), V_(2eff), V_(3eff) increases sharply, while the othertwo root mean square values remain constant, this does not indicate abreak in the connection of the neutral 6.

In order to confirm the suspected break in the connection of the neutral6, the processing unit 7A calculates the second quantity G2 by using thefollowing formula:

${{G2} = \sqrt{\frac{1}{3}\left( {{V_{1{eff}}V_{2{eff}}} + {V_{2{eff}}V_{3{eff}}} + {V_{3{eff}}V_{1{eff}}}} \right)}},$

where V_(1eff), V_(2eff) and V_(3eff) are respectively the root meansquare value of the first phase voltage, the root mean square value ofthe second phase voltage and the root mean square value of the thirdphase voltage.

The second quantity G2 is proposed because it is limited when a break inthe connection of the neutral 6 occurs. In the first embodiment, thepredetermined reference criterion which is defined by the processingunit 7A is therefore to check that the second quantity G2 is limitedbetween a lower limit BornInf and an upper limit BornSup:BorneInf≤G2≤BorneSup.

In order to correctly define BornInf and BornSup, the detection methodfurther comprises the steps of:

-   -   detecting whether the first, second and third phase voltages V₁,        V₂, V₃ are perfectly balanced, i.e., whether or not the root        mean square value of the first phase voltage V_(1eff), the root        mean square value of the second phase voltage V_(2eff) and the        root mean square value of the third phase voltage V_(3eff) are        all equal to the nominal root mean square value of the phase        voltage V_(nom) of the distribution network 1:

V _(1eff) =V _(nom) and V _(2eff) =V _(nom) and V _(3eff) =V _(nom),

-   -   if this condition is met, defining BorneInf and BorneSup as        follows:

${{BorneInf} = {V_{nom}{and}}}{{BorneSup} = {\sqrt{\frac{{2\sqrt{3}} + 1}{4}}{V_{nom}.}}}$

Otherwise, if said condition is not met, and therefore if:

-   -   V_(1eff)=α₁·V_(nom) and V_(2eff)=α₂·V_(nom), and        V_(3eff)=α₃·V_(nom), where a₁, a₂ and a₃ are real coefficients        such that α₁≠α₂≠α₃ and V_(nom) is said nominal root mean square        value:    -   BorneInf=min(B1, B2, B3, B4, B5, B6) and    -   BornSup=max(B1, B2, B3, B4, B5, B6),    -   where

${{B1} = {\sqrt{\frac{\sqrt{\left( {a_{1}^{2} + a_{2}^{2} + {a_{1}a_{2}}} \right)\left( {a_{1}^{2} + a_{3}^{2} + {a_{1}a_{3}}} \right)}}{3}}V_{nom}}},{{B2} = {\sqrt{\frac{\sqrt{\left( {a_{2}^{2} + a_{3}^{2} + {a_{2}a_{3}}} \right)\left( {a_{2}^{2} + a_{1}^{2} + {a_{2}a_{1}}} \right)}}{3}}V_{nom}}},{{B3} = {\sqrt{\frac{\left( {a_{3}^{2} + a_{1}^{2} + {a_{3}a_{1}}} \right)\left( {a_{3}^{2} + a_{2}^{2} + {a_{3}a_{2}}} \right)}{3}}V_{nom}}},$${{B4} = {\sqrt{\frac{\begin{matrix}{{2\sqrt{\left( {{4a_{1}^{2}} + a_{2}^{2} + a_{3}^{2} + {2a_{1}a_{2}} + {2a_{1}a_{3}} - {a_{2}a_{3}}} \right)\left( {a_{2}^{2} + a_{3}^{2} + {a_{2}a_{3}}} \right)}} +} \\\left( {a_{2}^{2} + a_{3}^{2} + {a_{2}a_{3}}} \right)\end{matrix}}{12}}V_{nom}}},$ ${{B5} = {\sqrt{\frac{\begin{matrix}{{2\sqrt{\left( {{4a_{2}^{2}} + a_{3}^{2} + a_{1}^{2} + {2a_{2}a_{3}} + {2a_{2}a_{1}} - {a_{3}a_{1}}} \right)\left( {a_{3}^{2} + a_{1}^{2} + {a_{3}a_{1}}} \right)}} +} \\\left( {a_{3}^{2} + a_{1}^{2} + {a_{3}a_{1}}} \right)\end{matrix}}{12}}V_{nom}}},$ ${B6} = {\sqrt{\frac{\begin{matrix}{{2\sqrt{\left( {{4a_{3}^{2}} + a_{1}^{2} + a_{2}^{2} + {2a_{3}a_{1}} + {2a_{3}a_{2}} - {a_{1}a_{2}}} \right)\left( {a_{1}^{2} + a_{2}^{2} + {a_{1}a_{2}}} \right)}} +} \\\left( {a_{1}^{2} + a_{2}^{2} + {a_{1}a_{2}}} \right)\end{matrix}}{12}}{V_{nom}.}}$

Annex 1 proposes a mathematical demonstration for determining theexpression of BornInf and BornSup.

If the second quantity G2 is limited between the lower limit BornInf andthe upper limit BornSup (i.e., if the second quantity G2 satisfies thepredetermined reference criterion), the detection method according tothe invention detects a break in the neutral connection at the time Tupstream of the first meter 7.

In the first embodiment, it is also possible to detect a break in theconnection of the neutral 6 when one phase from the first, second andthird phases 5A, 5B, 5C is disconnected. In this instance, the thirdphase 5C is considered to be disconnected. If a break occurs in theconnection of the neutral 6, there is a characteristic relation betweenthe root mean square value of the first phase voltage V_(1eff) and theroot mean square value of the second phase voltage V_(2eff).

If the first, second and third phase voltages V₁, V₂, V₃ are perfectlybalanced, i.e., if: V_(1eff)=V_(nom), and V_(2eff)=V_(nom) andV_(3eff)=V_(nom), the characteristic relation is the following:

-   -   V_(1eff)+V_(2eff)=√{square root over (3)}V_(nom), where V_(nom)        is the nominal root mean square value of the phase voltage.

If the first, second and third phase voltages V₁, V₂, V₃ are notperfectly balanced, i.e., if: V_(1eff)=α₁·V_(nom) andV_(2eff)=α₂·V_(nom) and V_(3eff)=α₃·V_(nom), (α₁≠α₂≠α₃) thecharacteristic relation is the following:

-   -   V_(1eff)+V_(2eff)=V_(nom)√{square root over (α₁ ²+α₂ ²+α₁α₂)},        where V_(nom) is the nominal root mean square value of the phase        voltage.

A more complete and detailed algorithm for implementing the detectionmethod according to the first embodiment of the invention is presentedin Annex 2.

According to a second embodiment, the detection method comprises thestep of evaluating the at least one second quantity which comprises asecond quantity A that is a function of an area of an actual triangle 19formed by the first, second and third phase voltages V₁, V₂, V₃ in theFresnel diagram.

The actual triangle 19 has a first edge constituted by the firstline-to-line voltage U₁₂, a second edge constituted by the secondline-to-line voltage U₂₃ and a third edge constituted by the thirdline-to-line voltage U₃₁.

The second quantity A is determined by the processing unit 7A by usingthe formula:

${A = {\frac{1}{2}\left( {{V_{1{eff}}V_{2{eff}}\sin\varphi_{12}} + {V_{2{eff}}V_{3{eff}}\sin\varphi_{23}} + {V_{3{eff}}V_{1{eff}}\sin\varphi_{31}}} \right)}},$

where A is the second quantity (which is the area of the actual triangle19), V_(1eff), V_(2eff) and V_(3eff) are respectively the root meansquare values of the first, second and third phase voltages, and φ₁₂ isthe first phase shift, φ₂₃ is the second phase shift and φ₃₁ is thethird phase shift. The first phase shift φ₁₂, the second phase shift φ₂₃and the third phase shift φ₃₁ are determined by the processing unit 7Abased on the first, second and third phase voltages V₁, V₂, V₃ via azero-crossing method and appropriate filtering.

The reference criterion is then that the second quantity A is such that:A_(ref)−ε₁≤A≤A_(ref)ε₁, where A_(ref) is a predetermined area of areference triangle 18 and ε₁ is a first predetermined measurementuncertainty (typically, +/−1% or +/−2%).

In reference to FIG. 5 , the area of the reference triangle 18 A_(ref)is actually the area of the triangle formed by the first, second andthird phase voltages V₁, V₂, V₃ in the Fresnel diagram in nominalconditions (i.e., when there is no break in the connection of theneutral 6).

Indeed, it is interesting to note that the area of the referencetriangle 18 and the area of the actual triangle 19 are similar in theevent of a break in the connection of the neutral 6. Therefore, bycomparing the second quantity A (the area of the actual triangle 19)with the predetermined area of the reference triangle 18 A_(ref), it ispossible to effectively determine whether a break in the connection ofthe neutral 6 has occurred.

In order to correctly define the area of the reference triangle 18A_(ref), the detection method further comprises the steps of:

-   -   detecting whether the first, second and third phase voltages V₁,        V₂, V₃ are perfectly balanced, i.e., whether or not the root        mean square value of the first phase voltage V left-, the root        mean square value of the second phase voltage V_(2eff) and the        root mean square value of the third phase voltage V_(3eff) are        all equal to the nominal root mean square value of the phase        voltage V_(nom), of the distribution network 1:

V _(1eff) =V _(nom) and V _(2eff) =V _(nom) and V _(3eff) =V _(nom),

-   -   if this condition is met, the area of the reference triangle 18        is evaluated by using the formula:

${A_{ref} = {\frac{3\sqrt{3}}{4}V_{nom}}},$

where V_(nom) is the nominal root mean square value of the phasevoltage.

Otherwise, if said condition is not met, and therefore if:

-   -   V_(1eff)=α₁·V_(nom) and V_(2eff)=α₂·V_(nom), and        V_(3eff)=α₃·V_(nom), where a₁, a₂ and a₃ are real coefficients        such that α₁≠α₂≠α₃ and V_(nom) is said nominal root mean square        value, the area of the reference triangle 18 is evaluated by        using the formula:

${A_{ref} = {\left( {{a_{1}a_{2}} + {a_{2}a_{3}} + {a_{3}a_{1}}} \right)\frac{\sqrt{3}}{4}V_{nom}}},$

where V_(nom) is said nominal root mean square value of the phasevoltage.

If the second quantity A satisfies the predetermined referencecriterion, the detection method according to the invention detects abreak in the neutral connection at the time T upstream of the firstmeter 7.

According to a third embodiment of the invention, the at least onesecond quantity comprises second quantities which comprise the firstline-to-line voltage U₁₂, the second line-to-line voltage U₂₃ and thethird line-to-line voltage U₃₁.

In reference to FIG. 5 , in the event of a break in the connection ofthe neutral 6, the first line-to-line voltage U₁₂, the secondline-to-line voltage U₂₃ and the third line-to-line voltage U₃₁ remainconstant.

The reference criterion checked by the processing unit 7A is thereforethat of checking whether:

Φ₁ε₂ ≤U ₁₂≤Φ₁+ε₂ and

Φ₂−ε₂ ≤U ₂₃≤Φ₂+ε₂ and

Φ₃−ε₂ ≤U ₃₁≤Φ₃+ε₂,

where Φ₁, Φ₂ and Φ₃ are respectively reference values of the first,second and third line-to-line voltages U₁₂, U₂₃, U₃₁ measured duringoperation at a reference time T₀ preceding the time T and ε₂ is a secondpredetermined measurement uncertainty (typically, +/−1% or +/−2%).

If the second quantities U₁₂, U₂₃ and U₃₁ satisfy the predeterminedreference criterion, the detection method according to the inventiondetects a break in the neutral connection at the time T upstream of thefirst meter 7.

According to a fourth embodiment of the invention, the detection methodcomprises the step of evaluating the at least one second quantity whichcomprises second quantities which comprise the first phase shift Φ₁₂,the second phase shift Φ₂₃ and the third phase shift Φ₃₁.

In reference to FIG. 5 , it is interesting to note that the first phaseshift Φ₁₂, the second phase shift Φ₂₃ and the third phase shift Φ₃₁ areno longer equal to 120° when a break in the connection of the neutral 6occurs.

The reference criterion defined by the processing unit 7A is thensatisfied if the first, second and third phase shifts Φ₁₂, φ₂₃, Φ₃₁ areeach non-zero and different to 120 degrees.

If the first, second and third phase shifts Φ₁₂, Φ₂₃, Φ₃₁ satisfy thepredetermined reference criterion, the detection method according to theinvention detects a break in the neutral connection at the time Tupstream of the first meter 7.

In the four embodiments of the invention disclosed above, if the secondquantity or quantities do not satisfy the predetermined referencecriterion, the detection method moves on to step E1′, which is similarto step E1, and possibly includes a step E2′ which is similar to stepE2.

In the four embodiments of the invention disclosed above, a suitablemargin of uncertainty, relating in particular to the accuracy of thevoltage sensors 7D of the first meter 7, is to be taken intoconsideration. Typically, the margin of uncertainty is in the region of+/−1% or +/−2%.

In the four embodiments of the invention disclosed above, theexpressions used for the definition of the predetermined referencecriterion (BornInf, BornSup, A_(ref), Φ₁, Φ₂ and Φ₃) could be determinedby the processing unit 7A by means of several successive calculations inorder to mitigate parasitic phenomena (such as a transient overvoltageof the shockwave or micro-cut-off type) that could decrease thereliability of the detection method according to the invention. The useof an averaging technique is quite feasible assuming that thedistribution network 1 is stable for a given time period.

Irrespective of the embodiment, the detection method according to theinvention is implemented continuously by the processing unit 7A of thefirst meter. The steps of said detection method are therefore repeatedregularly over time.

Advantageously, and irrespective of the embodiment, the detection methodaccording to the invention may further comprise the step of detecting abreak in connection of the neutral 6 when it has been detected that thesecond quantity satisfies the predetermined reference criterion apredetermined number of times (for example, 10 times), corresponding toconsecutive instances it is satisfied (i.e., consecutive implementationsof the detection method) spaced apart two by two in time by apredetermined duration (for example, 1 second). This increases thereliability of the detection method according to the invention.

Optionally, when a break in the neutral connection has effectively beendetected, the detection method may further comprise the step ofgenerating an alarm signal that can be timestamped in the memory 7B ofthe first meter 7 and/or that can be transmitted (via the communicationmodule 7C) to an item of equipment external to said first meter 7, forexample an information system of the distribution network 1.

Optionally, the alarm signal may be the displaying of a specific messageon a local display of the first meter 7 and/or the alarm signal may bethe illumination of an indicator light located on said first meter 7and/or the issuing of a sound signal by a loudspeaker of the first meter7.

Ideally, all of the electricity meters of the distribution network 1 aresimilar and are arranged to implement the detection method according tothe invention. Therefore, the whole of the distribution network 1 ismonitored correctly.

Optionally, when a break in the neutral connection has effectively beendetected and the imbalance between the first, second and third phasevoltages is considered to be too great, the detection method may furthercomprise the step, if said detection method is implemented in a specificmeter comprising a breaker, of opening said breaker in order to protectone or more electrical installations downstream of said specific meter.

Naturally, the invention is not limited to the described embodiments,but covers any variant that falls within the scope of the invention asdefined by the claims.

It is quite possible to freely combine the different embodimentsdisclosed above. Therefore, the processing unit 7A of the first meter 7may determine several second quantities, for example the second quantityG2 (disclosed in the first embodiment) and the second quantity A(disclosed in the second embodiment). This could increase thereliability of the detection method according to the invention.

The voltage sensors 7D of the first meter 7 comprising a first ADC 13, asecond ADC 14 and a third ADC 15 have been disclosed, but the voltagesensors 7D may quite possibly comprise a single ADC comprising threedistinct inputs each respectively acquiring the first phase voltage, thesecond phase voltage and the third phase voltage.

Annex 1: Formal Demonstration of the Relations Proposed for the FirstEmbodiment

The assumption for the present demonstration is that the first, secondand third phase voltages V₁, V₂, V₃ are perfectly balanced, i.e., thatthe root mean square values V_(1eff), V_(2eff), V_(3eff) are all equalto the nominal root mean square value of the phase voltage V_(nom). Thisis not necessarily accurate in practice, but can be used as a firstapproach.

It should be noted that voltages in the demonstration are in vector formand that, throughout the demonstration, they are complex root meansquare values.

In reference to FIG. 4 , once a break in the neutral connection hasoccurred, it is possible to write:

{right arrow over (V ₁₀)}−{right arrow over (V ₁)}={right arrow over (V_(ne))}

{right arrow over (V ₂₀)}−{right arrow over (V ₂)}={right arrow over (V_(ne))}

{right arrow over (V ₃₀)}−{right arrow over (V ₃)}={right arrow over (V_(ne))}

{right arrow over (I ₁)}+{right arrow over (I ₂)}+{right arrow over (I₃)}=0

I₁, I₂, I=are the electric currents flowing respectively through thefirst phase 5A, the second phase 5B and the third phase 5C.

${\frac{\overset{\rightarrow}{V_{10}}}{Z_{1}} + \frac{\overset{\rightarrow}{V_{20}}}{Z_{2}} + \frac{\overset{\rightarrow}{V_{30}}}{Z_{3}}} = {\overset{\rightarrow}{V_{ne}}\left( {\frac{1}{Z_{1}} + \frac{1}{Z_{2}} + \frac{1}{Z_{3}}} \right)}$

Using a first admittance Y₁, a second admittance Y₂ and a thirdadmittance Y₃ (Y₁=1/Z₁, Y₂=1/Z₂, Y₃=1/Z₃), the following applies:

{right arrow over (V ₁₀)}Y ₁+{right arrow over (V ₂₀)}Y ₂+{right arrowover (V ₃₀)}Y ₃={right arrow over (V _(ne))}(Y ₁ +Y ₂ +Y ₃)

Substitution {right arrow over (V_(ne))}: (1) is carried out

{right arrow over (V ₁₀)}Y ₁+{right arrow over (V ₂₀)}Y ₂+{right arrowover (V ₃₀)}Y ₃=({right arrow over (V ₁₀)}−{right arrow over (V ₁)})(Y ₁+Y ₂ +Y ₃)

Calculation of V₁

Since:

${V_{20} = {V_{10}\left( {{- \frac{1}{2}} - {j\frac{\sqrt{3}}{2}}} \right)}}{V_{30} = {V_{10}\left( {{- \frac{1}{2}} + {j\frac{\sqrt{3}}{2}}} \right)}}$

Based on the relation (1), the following applies:

${{V_{10}\left( {Y_{1} + {\left( {{- \frac{1}{2}} - {j\frac{\sqrt{3}}{2}}} \right)Y_{2}} + {\left( {{- \frac{1}{2}} + {j\frac{\sqrt{3}}{2}}} \right)Y_{3}}} \right)} = {\left( {V_{10} - V_{1}} \right)\left( {Y_{1} + Y_{2} + Y_{3}} \right)}}{{V_{10}\left\lbrack {\left( {Y_{1} - {\frac{1}{2}Y_{2}} - {\frac{1}{2}Y_{3}} - Y_{1} - Y_{2} - Y_{3}} \right) + {j\frac{\sqrt{3}}{2}\left( {Y_{3} - Y_{2}} \right)}} \right\rbrack} = {- {V_{1}\left( {Y_{1} + Y_{2} + Y_{3}} \right)}}}$$V_{1} = {V_{10}\left\lbrack {\frac{\left( {{\frac{3}{2}Y_{2}} + {\frac{3}{2}Y_{3}}} \right)}{\left( {Y_{1} + Y_{2} + Y_{3}} \right)} + {j\frac{\sqrt{3}}{2}\frac{\left( {Y_{2} - Y_{3}} \right)}{\left( {Y_{1} + Y_{2} + Y_{3}} \right)}}} \right.}$

Applying the modulus gives: (2)

$V_{1} = {V_{10}\sqrt{\left( \frac{\frac{3}{2}\left( {Y_{2} + Y_{3}} \right)}{\left( {Y_{1} + Y_{2} + Y_{3}} \right)} \right)^{2} + \left( {\frac{\sqrt{3}}{2}\frac{\left( {Y_{2} - Y_{3}} \right)}{\left( {Y_{1} + Y_{2} + Y_{3}} \right)}} \right)^{2}}}$

Calculation of V₂

Based on the relation (1), the following applies:

${V_{10}\left( {Y_{1} + {\left( {{- \frac{1}{2}} - {j\frac{\sqrt{3}}{2}}} \right)Y_{2}} + {\left( {{- \frac{1}{2}} + {j\frac{\sqrt{3}}{2}}} \right)Y_{3}}} \right)} = {\left( {V_{20} - V_{2}} \right)\left( {Y_{1} + Y_{2} + Y_{3}} \right)}$${{Since}:}{V_{20} = {V_{10}\left( {{- \frac{1}{2}} - {j\frac{\sqrt{3}}{2}}} \right)}}$

It follows that:

${V_{10}\left\lbrack {\left( {Y_{1} - {\frac{1}{2}Y_{2}} - {\frac{1}{2}Y_{3}} + {\frac{1}{2}Y_{1}} + {\frac{1}{2}Y_{2}} + {\frac{1}{2}Y_{3}}} \right) + {j\frac{\sqrt{3}}{2}\left( {Y_{3} - Y_{2} + Y_{1} + Y_{2} + Y_{3}} \right)}} \right\rbrack} = {- {V_{2}\left( {Y_{1} + Y_{2} + Y_{3}} \right)}}$$V_{2} = {V_{10}\left( {\frac{\left( {{- \frac{3}{2}}Y_{1}} \right)}{\left( {Y_{1} + Y_{2} + Y_{3}} \right)} - {j\frac{\sqrt{3}}{2}\frac{\left( {{2 \times Y_{3}} + Y_{1}} \right)}{\left( {Y_{1} + Y_{2} + Y_{3}} \right)}}} \right)}$

Applying the modulus gives: (3)

$V_{2} = {V_{10}\sqrt{\left( {- \frac{\frac{3}{2}Y_{1}}{\left( {Y_{1} + Y_{2} + Y_{3}} \right)}} \right)^{2} + \left( {{- \frac{\sqrt{3}}{2}}\frac{\left( {{2 \times Y_{3}} + Y_{1}} \right)}{\left( {Y_{1} + Y_{2} + Y_{3}} \right)}} \right)^{2}}}$

Calculation of V₃

Based on the relation (1), the following applies:

${V_{10}\left( {Y_{1} + {\left( {{- \frac{1}{2}} - {j\frac{\sqrt{3}}{2}}} \right)Y_{2}} + {\left( {{- \frac{1}{2}} + {j\frac{\sqrt{3}}{2}}} \right)Y_{3}}} \right)} = {\left( {V_{30} - V_{3}} \right)\left( {Y_{1} + Y_{2} + Y_{3}} \right)}$${{Since}:}{V_{30} = {V_{10}\left( {{- \frac{1}{2}} + {j\frac{\sqrt{3}}{2}}} \right)}}$

It follows that:

${V_{10}\left\lbrack {\left( {Y_{1} - {\frac{1}{2}Y_{2}} - {\frac{1}{2}Y_{3}} + {\frac{1}{2}Y_{1}} + {\frac{1}{2}Y_{2}} + {\frac{1}{2}Y_{3}}} \right) + {j\frac{\sqrt{3}}{2}\left( {Y_{3} - Y_{2} - Y_{1} - Y_{2} - Y_{3}} \right)}} \right\rbrack} = {- {V_{2}\left( {Y_{1} + Y_{2} + Y_{3}} \right)}}$$V_{3} = {V_{10}\left( {\frac{\left( {{- \frac{3}{2}}Y_{1}} \right)}{\left( {Y_{1} + Y_{2} + Y_{3}} \right)} + {j\frac{\sqrt{3}}{2}\frac{\left( {{2 \times Y_{2}} + Y_{1}} \right)}{\left( {Y_{1} + Y_{2} + Y_{3}} \right)}}} \right)}$

Applying the modulus gives: (4)

$V_{3} = {V_{10}\sqrt{\left( \frac{\frac{3}{2}Y_{1}}{\left( {Y_{1} + Y_{2} + Y_{3}} \right)} \right)^{2} + \left( {\frac{\sqrt{3}}{2}\frac{\left( {{2 \times Y_{2}} + Y_{1}} \right)}{\left( {Y_{1} + Y_{2} + Y_{3}} \right)}} \right)^{2}}}$

Using the relation (2), (3) and (4) and positing V₁₀=V_(nom) gives:

V ₁ V ₂ +V ₁ V ₃ +V ₂ V ₃ =V _(nom) ²ƒ(Y ₁ ,Y ₂ ,Y ₃)

Where:

${f\left( {Y_{1},Y_{2},Y_{3}} \right)} = {\sqrt{\begin{matrix}{\left( {\left( \frac{\frac{3}{2}\left( {Y_{2} + Y_{3}} \right)}{\left( {Y_{1} + Y_{2} + Y_{3}} \right)} \right)^{2} + \left( {\frac{\sqrt{3}}{2}\frac{\left( {Y_{2} - Y_{3}} \right)}{\left( {Y_{1} + Y_{2} + Y_{3}} \right)}} \right)^{2}} \right)\left( {\left( \frac{\frac{3}{2}Y_{1}}{\left( {Y_{1} + Y_{2} + Y_{3}} \right)} \right)^{2} +} \right.} \\\left. \left( {\frac{\sqrt{3}}{2}\frac{\left( {{2 \times Y_{3}} + Y_{1}} \right)}{\left( {Y_{1} + Y_{2} + Y_{3}} \right)}} \right)^{2} \right)\end{matrix}} + \sqrt{\begin{matrix}{\left( {\left( \frac{\frac{3}{2}\left( {Y_{2} + Y_{3}} \right)}{\left( {Y_{1} + Y_{2} + Y_{3}} \right)} \right)^{2} + \left( {\frac{\sqrt{3}}{2}\frac{\left( {Y_{2} - Y_{3}} \right)}{\left( {Y_{1} + Y_{2} + Y_{3}} \right)}} \right)^{2}} \right)\left( {\left( {- \frac{\frac{3}{2}Y_{1}}{\left( {Y_{1} + Y_{2} + Y_{3}} \right)}} \right)^{2} +} \right.} \\\left. \left( {\frac{\sqrt{3}}{2}\frac{\left( {{2 \times Y_{2}} + Y_{1}} \right)}{\left( {Y_{1} + Y_{2} + Y_{3}} \right)}} \right)^{2} \right)\end{matrix}} + \sqrt{\begin{matrix}{\left( {\left( \frac{\frac{3}{2}Y_{1}}{\left( {Y_{1} + Y_{2} + Y_{3}} \right)} \right)^{2} + \left( {\frac{\sqrt{3}}{2}\frac{\left( {{2 \times Y_{3}} + Y_{1}} \right)}{\left( {Y_{1} + Y_{2} + Y_{3}} \right)}} \right)^{2}} \right)\left( {\left( {- \frac{\frac{3}{2}Y_{1}}{\left( {Y_{1} + Y_{2} + Y_{3}} \right)}} \right)^{2} +} \right.} \\\left. \left( {\frac{\sqrt{3}}{2}\frac{\left( {{2 \times Y_{2}} + Y_{1}} \right)}{\left( {Y_{1} + Y_{2} + Y_{3}} \right)}} \right)^{2} \right)\end{matrix}}}$ f(Y₁, Y₂, Y₃)=$\frac{1}{4\left( {Y_{1} + Y_{2} + Y_{3}} \right)^{2}}\left( {\sqrt{\left( {{9\left( {Y_{2} + Y_{3}} \right)^{2}} + {3\left( {Y_{3} - Y_{2}} \right)^{2}}} \right)\left( {{9Y_{1}^{2}} + {3\left( {Y_{1} + {2Y_{3}}} \right)^{2}}} \right)} +} \right.$$\sqrt{\left( {{9\left( {Y_{2} + Y_{3}} \right)^{2}} + {3\left( {Y_{3} - Y_{2}} \right)^{2}}} \right)\left( {{9Y_{1}^{2}} + {3\left( {Y_{1} + {2Y_{2}}} \right)^{2}}} \right)} +$$\left. \sqrt{\left( {{9Y_{1}^{2}} + {3\left( {Y_{1} + {2Y_{3}}} \right)^{2}}} \right)\left( {{9Y_{1}^{2}} + {3\left( {Y_{1} + {2Y_{2}}} \right)^{2}}} \right)} \right)$${f\left( {Y_{1},Y_{2},Y_{3}} \right)} = {\frac{3}{\left( {Y_{1} + Y_{2} + Y_{3}} \right)^{2}}\left( {\sqrt{\left( {Y_{2}^{2} + {Y_{2}Y_{3}} + Y_{3}^{2}} \right)\left( {Y_{1}^{2} + {Y_{1}Y_{3}} + Y_{3}^{2}} \right)} +} \right.}$$\left. {\sqrt{\left( {Y_{2}^{2} + {Y_{2}Y_{3}} + Y_{3}^{2}} \right)\left( {Y_{1}^{2} + {Y_{1}Y_{2}} + Y_{2}^{2}} \right)} + \sqrt{\left( {Y_{1}^{2} + {Y_{1}Y_{3}} + Y_{3}^{2}} \right)\left( {Y_{1}^{2} + {Y_{1}Y_{2}} + Y_{2}^{2}} \right)}} \right)$

Studying the Limit Cases:

Case 1: Y₁=Y₂=Y₃=Y:

${f\left( {Y_{1},Y_{2},Y_{3}} \right)} = {{\frac{3}{9Y^{2}}\left( {\sqrt{9Y^{4}} + \sqrt{9Y^{4}} + \sqrt{9Y^{4}}} \right)} = {{\frac{3}{9Y^{2}}9Y^{2}} = 3}}$

Since:

V ₁ V ₂ +V ₁ V ₃ +V ₂ V ₃ =V _(nom) ²ƒ(Y ₁ ,Y ₂ ,Y ₃)

It follows that (the limit case studied here constituting the lowerlimit)—V_(i) and V_(j) being respectively the phase voltages of the ithphase and jth phase:

${\sum\limits_{i \neq j}{V_{i}V_{j}}} \geq {3V_{nom}^{2}}$

Hence:

$V_{nom} \leq \sqrt{\frac{{\sum}_{i \neq j}V_{i}V_{j}}{3}}$

Case 2: Y₁=Y₂=Y>>Y₃=y:

${f\left( {Y_{1},Y_{2},Y_{3}} \right)} = {{\frac{3}{\left( {2Y} \right)^{2}}\left( {\sqrt{\left( Y^{2} \right)\left( Y^{2} \right)} + \sqrt{\left( Y^{2} \right)\left( {3Y^{2}} \right)} + \sqrt{\left( Y^{2} \right)\left( {3Y^{2}} \right)}} \right)} = \frac{3\left( {{2\sqrt{3}} + 1} \right)}{4}}$

Since:

V ₁ V ₂ +V ₁ V ₃ +V ₂ V ₃ =V _(nom) ²ƒ(Y ₁ ,Y ₂ ,Y ₃)

It follows that (the limit case studied here constituting the upperlimit):

${\sum\limits_{i \neq j}{V_{i}V_{j}}} \leq {V_{nom}^{2} \times \frac{3\left( {{2\sqrt{3}} + 1} \right)}{4}}$

Hence:

$\sqrt{\frac{{\sum}_{i \neq j}V_{i}V_{j}}{3}} \leq {\sqrt{\frac{{2\sqrt{3}} + 1}{4}}V_{nom}}$

We have shown, by studying limit cases, that the sum of the V_(i)V_(j)remained very precisely limited between V_(nom) and 1.056V_(nom) in theevent of a break in the neutral connection:

${{BorneInf} \leq \sqrt{\frac{{\sum}_{i \neq j}V_{i}V_{j}}{3}} \leq {BorneSup}}{{BorneInf} = V_{nom}}{{BorneSup} = {\sqrt{\frac{{2\sqrt{3}} + 1}{4}}{V_{nom}\left( {= {1.056V_{nom}}} \right)}}}$

The proposed formula can then be generalized, by the same calculationprocess, to take into account initial conditions for which thedistribution network does not have perfectly balanced phase voltages, bypositing:

V_(1eff)=α₁·V_(nom) and V_(2eff)=α₂·V_(nom) and V_(3eff)=α₃·V_(nom),where a₁, a₂ and a₃ are real coefficients such that α₁≠α₂≠α₃.

The fact is that, considering the effect of the initial imbalancesbetween the phase voltages coming from the distribution network, theasymptotic study cases to be considered (which make it possible toobtain a bounded limit of the result expected in the event of a break inthe neutral connection) increase from 2 to 6, due to loss of symmetry.

The 6 limits are given by:

${{B1} = {\sqrt{\frac{\sqrt{\left( {a_{1}^{2} + a_{2}^{2} + {a_{1}a_{2}}} \right)\left( {a_{1}^{2} + a_{3}^{2} + {a_{1}a_{3}}} \right)}}{3}}V_{nom}}},{{B2} = {\sqrt{\frac{\sqrt{\left( {a_{2}^{2} + a_{3}^{2} + a_{2} + a_{3}} \right)\left( {a_{2}^{2} + a_{1}^{2} + {a_{2}a_{1}}} \right)}}{3}}V_{nom}}},{{B3} = {\sqrt{\frac{\sqrt{\left( {a_{3}^{2} + a_{1}^{2} + {a_{3}a_{1}}} \right)\left( {a_{3}^{2} + a_{2}^{2} + {a_{3}a_{2}}} \right)}}{3}}V_{nom}}},$${{B4} = {\sqrt{\frac{\begin{matrix}{{2\sqrt{\left( {{4a_{1}^{2}} + a_{2}^{2} + a_{3}^{2} + {2a_{1}a_{2}} + {2a_{1}a_{3}} - {a_{2}a_{3}}} \right)\left( {a_{2}^{2} + a_{3}^{2} + {a_{2}a_{3}}} \right)}} +} \\\left( {a_{2}^{2} + a_{3}^{2} + {a_{2}a_{3}}} \right)\end{matrix}}{12}}V_{nom}}},$ B5= ${\sqrt{\frac{\begin{matrix}{{2\sqrt{\left( {{4a_{2}^{2}} + a_{3}^{2} + a_{1}^{2} + {2a_{2}a_{3}} + {2a_{2}a_{1}} - {a_{3}a_{1}}} \right)\left( {a_{3}^{2} + a_{1}^{2} + {a_{3}a_{1}}} \right)}} +} \\\left( {a_{3}^{2} + a_{1}^{2} + {a_{3}a_{1}}} \right)\end{matrix}}{12}}V_{nom}},$ ${B6} = {\sqrt{\frac{\begin{matrix}{{2\sqrt{\left( {{4a_{3}^{2}} + a_{1}^{2} + a_{2}^{2} + {2a_{3}a_{1}} + {2a_{3}a_{2}} - {a_{1}a_{2}}} \right)\left( {a_{1}^{2} + a_{2}^{2} + {a_{1}a_{2}}} \right)}} +} \\\left( {a_{1}^{2} + a_{2}^{2} + {a_{1}a_{2}}} \right)\end{matrix}}{12}}{V_{nom}.}}$

And Finally:

Where:

${BorneInf} \leq \sqrt{\frac{{\sum}_{i \neq j}V_{i}V_{j}}{3}} \leq {BorneSup}$

BorneInf=min(B1, B2, B3, B4, B5, B6) and BornSup=max(B1, B2, B3, B4, B5,B6).

Annex 2: Algorithm proposed for the first embodiment // init Un = 230 U[]=[230;230;230] PhaseConnectStatus[ ] = [UNKNOWN; UNKNOWN; UNKNOWN]NeutraConnectStatus = UNKNOWN // param Vmin = 60 Tol = 10% N = 2AlphaThreshold = 20% DevThresholdLow = 10% DevThresholdHigh = 20%MarginLow = 0.5% MarginHigh = 0.5% MarginLow_bis = 2% MarginHigh_bis =2% MarginAdd = 5% Loop every second  For i = 1 to 3   cnt_unknwn = 0  If (V[i] < Vmin)    PhaseConnectStatus[i] = UNKNOWN    cnt_unknwn ++  Else    PhaseConnectStatus[i] = CONNECTED   End If  End For  For i = 1to 3   If V[i] < (1−Tol).Un    Vtmp = U[i]   Elsif V[i] > (1+Tol).Un   Vtmp = U[i]   Else    Vtmp = V[i]   End If   U[i] = Vtmp/N +(N−1).U[i]/N   a[i] = U[i]/Un   Dev[i] = V[i]/U[i]  End For  If(cnt_unknwn >= 2)   For i = 1 to 3    If (PhaseConnectStatus[i] =UNKNOWN)     PhaseConnectStatus[i] = DISCONNECTED    End If   End For End If  If (cnt_unknwn < 2)   Alpha = Max(V[ ])/MIN(V[ ])   If(((Alpha−1) > AlphaThreshold) AND (((1−MIN(Dev[ ])) > DevThresholdLow)OR ((MAX(Dev[ ])−1) > DevThresholdHigh)))    Vref = 0    For i = 1 to 3    j = i+1 %3     k = i+2 %3     Vref = Vref + V[i].V[j]/3    Factor1[i] = SQRT((a[i]²+a[j]²+a[i].a[j]).(a[i]²+a[k]²+a[i].a[k]))    Factor2[i] = (2.SQRT((4.a[i]²+a[j]²+a[k]²+2.a[i].a[j]+2.a[i].a[k]−a[j].a[k]).(a[j]²+a[k]²+a[j].a[k]))+(a[j]²+a[k]²+a[j].a[k]))/4    EndFor    Vref = SQRT(Vref)    Ulim1[i] = Un.SQRT(Factor1[i]/3)    Ulim2[i]= Un.SQRT(Factor2[i]/3)    If (NeutralConnectStatus = DISCONNECTED)    MarginL = MarginLow + MarginAdd     MarginH = MarginHigh + MarginAdd   Else     MarginL = MarginLow     MarginH = MarginHigh    End If    If((Vref > (1−MarginL).MIN(MIN(Ulim1[ ]),MIN(Ulim2[ ]))) AND (Vref <(1+MarginH).MAX(MAX(Ulim1[ ]),MAX(Ulim2[ ]))))     NeutralConnectStatus= DISCONNECTED     For i = 1 to 3      If (PhaseConnectStatus[i] =UNKNOWN)       PhaseConnectStatus[i] = CONNECTED      End If     End For   Else     NeutralConnectStatus = CONNECTED     For i = 1 to 3      If(PhaseConnectStatus[i] = UNKNOWN)       PhaseConnectStatus[i] =DISCONNECTED       NeutralConnectStatus = UNKNOWN      End If     EndFor    End If   End If   If (NeutralConnectStatus = UNKNOWN)    Vref_bis= 0    For i = 1 to 3     If (PhaseConnectStatus[i] = CONNECTED)     Vref_bis = Vref_bis + V[i]     End If    End For    Vref_bis =Vref_bis/2    For i = 1 to 3     a_bis[i] = 0     If(PhaseConnectStatus[i] = CONNECTED)      a_bis[i] = a[i]     End If   End For    Factor_bis = 0    For i = 1 to 3     j = i+1 %3    Factor_bis = Factor_bis + a_bis[i]² + a_bis[i].a_bis[j]    End For   Ulim_bis = Un.SQRT(Factor_bis)/2    If (NeutralConnectStatus =DISCONNECTED)     MarginL_bis = MarginLow_bis + MarginAdd    MarginH_bis = MarginHigh_bis + MarginAdd    Else     MarginL =MarginLow     MarginH = MarginHigh    End If    If ((Vref_bis >(1−MarginL_bis).Ulim_bis) AND (Vref_bis < (1+MarginH_bis).Ulim_bis))    NeutralConnectStatus = DISCONNECTED    Else     NeutralConnectStatus= CONNECTED    End If   End If  End If  End Loop

1. A method for detecting a break in connection of a neutral of athree-phase electricity network, the detection method being implementedat least partially in a processing unit of an item of electricalequipment connected to the electricity network, and comprising thesteps, repeated regularly, of: —acquiring, at a time T, a first phasevoltage (V₁) measured between a first phase of the three-phaseelectricity network and the neutral, a second phase voltage (V₂)measured between a second phase (5B) and the neutral, and a third phasevoltage (V₃) measured between a third phase and the neutral, the first,second and third phase voltages being measured by voltage sensors of theitem of electrical equipment; evaluating a first quantity representativeof a ratio between a maximum phase voltage and a minimum phase voltagefrom the first, second and third phase voltages; if the first quantityis greater than a predetermined threshold: evaluating, based on thefirst, second and third phase voltages, at least a second quantityrepresentative of a current balance between said first, second and thirdphase voltages; detecting a break in connection of the neutral at thetime T when the at least one second quantity satisfies a predeterminedreference criterion.
 2. The detection method according to claim 1,wherein the at least one second quantity comprises a second quantitythat is a function of a sum of pairwise products of root mean squarevalues of the first, second and third phase voltages.
 3. The detectionmethod according to claim 2, wherein said second quantity G2 is equalto:${{G2} = \sqrt{\frac{1}{3}\left( {{V_{1{eff}}V_{2{eff}}} + {V_{2{eff}}V_{3{eff}}} + {V_{3{eff}}V_{1{eff}}}} \right)}},$where V_(1eff), V_(2eff) and V_(3eff) are respectively a root meansquare value of the first phase voltage (V₁), a root mean square valueof the second phase voltage (V₂) and a root mean square value of thethird phase voltage (V₃), the predetermined reference criterion beingthat:BorneInf≤G2≤BorneSup.
 4. The detection method according to claim 3,further comprising the steps of: detecting whether:V _(1eff) =V _(nom) and V _(2eff) =V _(nom) and V _(3eff) =V _(nom),where V_(nom) is a nominal root mean square value of the phase voltageof the electricity network; if this condition is met, defining BorneInfand BorneSup as follows:${BorneInf} = {{V_{nom}{and}{BorneSup}} = {\sqrt{\frac{{2\sqrt{3}} + 1}{4}}{V_{nom}.}}}$5. The detection method according to claim 4, wherein, if said conditionis not met, and if: V_(1eff)=α₁·V_(nom) and V_(2eff)=α₂·V_(nom) andV_(3eff)=α₃·V_(nom), where a₁, a₂ and a₃ are real coefficients suchthat: BorneInf=min(B1, B2, B3, B4, B5, B6) and BornSup=max (B1, B2, B3,B4, B5, B6), where${{B1} = {\sqrt{\frac{\sqrt{\left( {a_{1}^{2} + a_{2}^{2} + {a_{1}a_{2}}} \right)\left( {a_{1}^{2} + a_{3}^{2} + {a_{1}a_{3}}} \right)}}{3}}V_{nom}}},{{B2} = {\sqrt{\frac{\sqrt{\left( {a_{2}^{2} + a_{3}^{2} + a_{2} + a_{3}} \right)\left( {a_{2}^{2} + a_{1}^{2} + {a_{2}a_{1}}} \right)}}{3}}V_{nom}}},{{B3} = {\sqrt{\frac{\sqrt{\left( {a_{3}^{2} + a_{1}^{2} + {a_{3}a_{1}}} \right)\left( {a_{3}^{2} + a_{2}^{2} + {a_{3}a_{2}}} \right)}}{3}}V_{nom}}},$${{B4} = {\sqrt{\frac{\begin{matrix}{{2\sqrt{\left( {{4a_{1}^{2}} + a_{2}^{2} + a_{3}^{2} + {2a_{1}a_{2}} + {2a_{1}a_{3}} - {a_{2}a_{3}}} \right)\left( {a_{2}^{2} + a_{3}^{2} + {a_{2}a_{3}}} \right)}} +} \\\left( {a_{2}^{2} + a_{3}^{2} + {a_{2}a_{3}}} \right)\end{matrix}}{12}}V_{nom}}},$ B5= ${\sqrt{\frac{\begin{matrix}{{2\sqrt{\left( {{4a_{2}^{2}} + a_{3}^{2} + a_{1}^{2} + {2a_{2}a_{3}} + {2a_{2}a_{1}} - {a_{3}a_{1}}} \right)\left( {a_{3}^{2} + a_{1}^{2} + {a_{3}a_{1}}} \right)}} +} \\\left( {a_{3}^{2} + a_{1}^{2} + {a_{3}a_{1}}} \right)\end{matrix}}{12}}V_{nom}},$ ${B6} = {\sqrt{\frac{\begin{matrix}{{2\sqrt{\left( {{4a_{3}^{2}} + a_{1}^{2} + a_{2}^{2} + {2a_{3}a_{1}} + {2a_{3}a_{2}} - {a_{1}a_{2}}} \right)\left( {a_{1}^{2} + a_{2}^{2} + {a_{1}a_{2}}} \right)}} +} \\\left( {a_{1}^{2} + a_{2}^{2} + {a_{1}a_{2}}} \right)\end{matrix}}{12}}{V_{nom}.}}$
 6. The detection method according toclaim 1, wherein the at least one second quantity comprises a secondquantity that is a function of an area of an actual triangle (19) formedby the first, second and third phase voltages in the Fresnel diagram. 7.The detection method according to claim 6, wherein the area of theactual triangle is determined by using the formula:${A = {\frac{1}{2}\left( {{V_{1{eff}}V_{2{eff}}\sin\varphi_{12}} + {V_{2{eff}}V_{3{eff}}\sin\varphi_{23}} + {V_{3{eff}}V_{1{eff}}\sin\varphi_{31}}} \right)}},$where A is the second quantity, V_(1eff), V_(2eff) and V_(3eff) arerespectively the root mean square values of the first, second and thirdphase voltages, and Φ₁₂ is a first phase shift between the first phasevoltage and the second phase voltage, φ₂₃ is a second phase shiftbetween the second phase voltage and the third phase voltage and φ₃₁ isa third phase shift between the third phase voltage and the first phasevoltage, the reference criterion then being that the second quantity issuch that:A _(ref)−ε₁ ≤A≤A _(ref)+ε₁ where A_(ref) is an area of a referencetriangle and ε₁ is a first predetermined measurement uncertainty.
 8. Thedetection method according to claim 7, wherein, if the first, second andthird phase voltages are perfectly balanced, the area of thepredetermined reference triangle (18) is evaluated by using the formula:${A_{ref} = {\frac{3\sqrt{3}}{4}V_{nom}}},$ where A_(ref) is the area ofthe reference triangle and V_(nom) is the nominal root mean square valueof the phase voltage of the electricity network.
 9. The detection methodaccording to claim 1, wherein the at least one second quantity comprisessecond quantities which comprise a first line-to-line voltage U₁₂representative of a difference between the first phase voltage and thesecond phase voltage, a second line-to-line voltage U₂₃ representativeof a difference between the second phase voltage and the third phasevoltage and a third line-to-line voltage U₃₁ representative of adifference between the third phase voltage and the first phase voltage,the reference criterion then being that:φ₁−ε₂ ≤U ₁₂≤φ₁+ε₂ andφ₂−ε₂ ≤U ₂₃≤φ₂+ε₂ andφ₃−ε₂ ≤U ₃₁≤φ₃+ε₂, where φ₁, φ₂ and φ₃ are reference values of thefirst, second and third line-to-line voltages measured during operationat a reference time T₀ preceding the time T and ε₂ is a secondpredetermined measurement uncertainty.
 10. The detection methodaccording to claim 1, wherein the at least one second quantity comprisessecond quantities that comprise a first phase shift between the firstphase voltage (V₁) and the second phase voltage (V₂), a second phaseshift between the second phase voltage (V₂) and the third phase voltage(V₃) and a third phase shift between the third phase voltage (V₃) andthe first phase voltage (V₁), the predetermined reference criterion thenbeing that the first, second and third phase shifts are each non-zeroand different to 120 degrees.
 11. The detection method according toclaim 1, further comprising the step of detecting a break in the neutralconnection when it has been detected that the second quantity satisfiesthe predetermined reference criterion a predetermined number of times,corresponding to consecutive instances it is satisfied, spaced apart twoby two in time by a predetermined duration.
 12. The detection methodaccording to claim 1, wherein, when a break in the neutral connectionhas been detected, the method further comprises the step of generatingan alarm signal that can be timestamped in a memory (7B) of the item ofelectrical equipment and/or that can be transmitted to an item ofequipment external to said item of electrical equipment.
 13. An item ofelectrical equipment comprising voltage sensors and a processing unitarranged to implement the detection method according to claim
 1. 14. Theitem of electrical equipment according to claim 13, the item ofelectrical equipment being an electricity meter.
 15. (canceled)
 16. Anon-transitory computer-readable storage medium on which a computerprogram comprising instructions that cause an item of electricalequipment comprising voltage sensors and a processing unit arranged toimplement the detection method according to claim 1 is stored.